Power transistors such as power MOS field effect transistors (MOSFETs), insulated gate bipolar transistors (IGBTs) or the like are often implemented as so-called vertical transistors. The term “vertical” refers to the direction of load current flow through the semiconductor die, which is vertically with respect to a top surface of the semiconductor die. Furthermore, such power transistors are usually composed of a plurality of transistor cells and, for each cell, the gate electrode is arranged in a so-called “trench”, which extends from the top surface vertically into the semiconductor die. This class of power transistors is usually referred to as “trench transistors”.
The semiconductor die (chip), in which a trench transistor is integrated, may include further circuitry to provide further functions besides its main function of being an electronic switch. For example, the chip may further include a temperature sensor and respective sense circuitry to measure the temperature of the transistor and to generate a temperature signal (i.e. a current or a voltage signal representing the temperature present at the location of the temperature sensor). The temperature signal may be used, for example, in an over-temperature or an overload protection circuit, which may be needed in order to protect the transistor against thermal breakdown. Further circuitry may be included in the chip to provide other functions such as current sensing, over-current protection, a digital bus interface (e.g. Serial Peripheral Interface, SPI) etc.
Common manufacturing technologies allow two wiring layers arranged on top of the semiconductor body, wherein the first wiring layer is usually formed by polycrystalline silicon and the second wiring layer is usually formed by a metal (e.g. aluminum). The two wiring layers are used to interconnect the individual circuit components integrated in the semiconductor die to form the desired electronic circuit. On top of the wiring layers (and isolated therefrom) a further metal layer is provided, which is comparably thick and sometimes referred to as “power metal layer”. This metal layer is used as contact layer (also acting as bond pad) to contact an external load terminal (e.g. source or collector terminal of the power transistor) with the chip.
The mentioned temperature sensors are usually arranged close to or within (e.g. in the center of) an array of transistor cells, which compose the power transistor, and usually the temperature sensor is connected to the respective sense circuit via strip lines (sense lines) formed the mentioned wiring layers on top of the semiconductor body. As the sense circuit may be formed in the semiconductor chip aside from the array of transistor cells the strip lines between sense circuit and temperature sensor may be comparably long, e.g. 300 μm or even more.
As the (patterned) wiring layers and the top metal layer forming the contact layer for an external load terminal are substantial parallel (coplanar) and separated by comparably thin insulation layers, a significant capacitive coupling occurs, particularly between the contact layer and the subjacent wiring layer. This capacitive coupling (due to parasitic capacitances between the wiring layers and the contact layer) results in a significant sensitivity to “direct power injection” (DPI). Particularly, when the power transistor is an n-channel device operated as high-side switch the electric potential (and thus the voltage) of the (power) contact layer will (during a switching operation) rapidly change from zero (ground potential) to approximately the upper supply potential and vice versa. This will cause displacement currents in the sense lines and have a negative impact on the temperature sensing as displacement currents may result in distortions of the temperature signal.
In view of the above, there is a need for an improved semiconductor switch with an integrated temperature sensor.